The question is no longer "whether" but "when" the Federal Reserve will begin to normalize monetary policy, but Boston Fed President Eric Rosengren advised patience and said it is impossible to know now when economic conditions will sufficiently lift for the Fed to raise rates.

Speaking at the American Economic Association’s annual meeting on a panel titled "Monetary Policy Normalization: Graceful Exit or Bumpy Ride?" Rosengren said the tightening of short-term interest rates depends on inflation, unemployment, wage growth and the global economy.

"Any change in policy, monetary or otherwise, has the potential for unanticipated effects. In assessing the potential consequences of a patient monetary policy response, for example, some observers worry that such a policy entails significant risks of overshooting full employment, overheating financial markets or even causing undesirably high inflation," Rosengren said.

He noted that the last time the Federal Open Market Committee (FOMC) raised rates following a recession, in June 2004, unemployment was 5.6 percent and inflation was 2.8 percent, compared with 5.8 percent unemployment and 1.2 percent inflation today.

"Some worry that patience will mean deferring the first rate increase until well past the arrival of economic conditions that historically result in tightening, but I would point out that we have some way to go before reaching those conditions, and so have not been unusually patient as yet," he said.

Rosengren said the complexity of monetary policy normalization is more pronounced than in 1994 and 2004, when the FOMC also raised rates following a recession.

"The low inflation rates experienced globally may also allow for a more gradual normalization process than typically occurs. With so little wage and price pressure, and relatively slow productivity growth, it is possible that rates may not normalize at the same level they were prior to the financial crisis," he said. "However… the fact that discussion of policy normalization is now appropriate is a welcome change from discussions of monetary policy over the past six years."